Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysed oxidation ... · role of platinum group metal ions as catalysts in N-haloamine oxidation of Gly-Gly. The studies on the use of platinum

Research Article

844

Received: 21 October 2007, Revised: 2 February 2008, Accepted: 25 March 2008, Published online in Wiley InterScience: 2 June 2008

(www.interscience.wiley.com) DOI 10.1002/poc.1379

Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysedoxidation of glycyl–glycine by sodiumN-chloro-p-toluenesulfonamide: comparativemechanistic aspects and kinetic modellingR. V. Jagadeesha and Puttaswamya*

The Ru(III)/Os(VIII)/Pd(II)/Pt(IV)-catalysed kinetics o

J. Phys. Or

f oxidation of glycyl–glycine (Gly-Gly) by sodium N-chloro-p-toluenesulfonamide (chloramine-T; CAT) in NaOH medium has been investigated at 308K. The stoichiometry andoxidation products in each case were found to be the same but their kinetic patterns observed are different.Under comparable experimental conditions, the oxidation-kinetics and mechanistic behaviour of Gly-Gly with CATin NaOH medium is different for each catalyst and obeys the underlying rate laws:

Rate¼ k [CAT]t [Gly-Gly]0 [Ru(III)][OHS]x

Rate¼ k [CAT]t[Gly-Gly]x [Os(VIII)]y[OHS]z

Rate¼ k [CAT]t[Gly-Gly]x [Pd(II)][OHS]y

Rate¼ k [CAT]t[Gly-Gly]0 [Pt(IV)]x[OHS]y

Here, Ts¼CH3C6H4SO2� and x, y, z< 1 in all the cases. The anion of CAT, CH3C6H4SO2NClS, has been postulated as

the common reactive oxidising species in all the cases. Under comparable experimental conditions, the relative abilityof these catalysts towards oxidation of Gly-Gly by CATare in the order: Os(VIII) > Ru(III) > Pt(IV) > Pd(II). This trendmaybe attributed to the different d-electronic configuration of the catalysts. Further, the rates of oxidation of all the fourcatalysed reactions have been compared with uncatalysed reactions, under identical experimental conditions. It wasfound that the catalysed reaction rates are 7- to 24-fold faster. Based on the observed experimental results, detailedmechanistic interpretation and the related kinetic modelling have been worked out for each catalyst. Copyright �2008 John Wiley & Sons, Ltd.

Keywords: Ru(III)/Os(VIII)/Pd(II)/Pt(IV) catalysis; oxidation-kinetics; glycyl–glycine; chloramine-T; alkaline medium

* Correspondence to: Puttaswamy, Department of Chemistry, Central College

Campus, Bangalore University, Bangalore-560 001, India.

E-mail: [email protected]

a R. V. Jagadeesh, Puttaswamy

Department of Chemistry, Central College Campus, Bangalore University,

Bangalore-560 001, India

INTRODUCTION

The chemistry of N-haloamines is of interest due to their diversechemical behaviour.[1] These compounds resemble hypohalites intheir oxidative behaviour and they are more stable thanhypohalites.[2,3] Consequently, these compounds react with awide range of functional groups and effect a variety of molecularchanges.[1] The prominent member of this class of compoundsis sodium N-chloro-4-methylbenzenesulfonamide, commonlyknown as chloramine-T (p-CH3C6H4SO2NClNa�3H2O and abbre-viated as CAT). Mechanistic aspects of many of its reactions havebeen well documented.[1,4–6] However, a very little information isavailable in the literature on CAT reactions with dipeptides.Dipeptides are useful biomaterials in many biological,

pharmaceutical, analytical and synthetic applications. Glycyl–Glycine (Gly-Gly) is the first member of the dipeptide series anda review of literature reveals that there are many reports onthe kinetics of its hydrolysis.[7,8] It has also been oxidised bymanganese-(III),[9] bromamine-T,[10] bromamine-B[11] in acidmedium. However, literature survey revealed that there are noefforts being made from the kinetic and mechanistic viewpointson the oxidation of Gly-Gly by N-haloamines in alkaline medium.Hence, there was a need for understanding the oxidationmechanism of this dipeptide in alkaline medium, so that this

g. Chem. 2008, 21 844–858 Copyright �

study is expected to throw some light on the mechanism ofmetabolic conversion of Gly-Gly in the biological systems. Hence,the present kinetic study gives an impetus, as the Gly-Gly is abiologically important substrate. Also, no one has examined therole of platinum group metal ions as catalysts in N-haloamineoxidation of Gly-Gly.The studies on the use of platinum group metal ions, either

alone or binary mixtures, as catalysts in many redox reactions,have been gaining interest.[12] Common platinum group metalions employed in homogeneous catalysis involving redoxreactions are Ruthenium (III) chloride (Ru(III)), Osmium(VIII) oxide(Os(VIII)), Palladium(II) chloride (Pd(II)), Platinum(IV) chloride(Pt(IV)), Iridium(III) chloride (Ir(III)) and Rhodium(III) chloride(Rh(III)). The mechanisms of these catalysis are quite complicateddue to the formation of different intermediate complexes, freeradicals and differing oxidising states of these catalysts.[13,14]

2008 John Wiley & Sons, Ltd.

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE

Although many complexes of these metal ions with variousorganic and inorganic substances have been reported, anextensive literature survey reveals the absence of comprehensivestudies on the oxidation-kinetics and mechanism of dipeptidesby any of these catalysts. Consequently, such studies provide aninsight into the interaction of metal ions with the substrate inbiological systems.This background instigated us to carry out the title reaction in

order to interpret the mechanism and to understand the catalyticredox chemistry of Gly-Gly in the presence of platinum groupmetal ions. Preliminary experimental results revealed that thereactions of Gly-Gly with CAT in alkaline medium without acatalyst were sluggish, but the reactions become facile in thepresence of a micro quantity of platinum group metal ionslike Ru(III), Os(VIII), Pd(II) and Pt(IV). The main objectives of thepresent investigation are to: (i) elucidate plausible mechanisms,(ii) deduce appropriate rate laws, (iii) ascertain the variousreactive species, (iv) assess the relative rates of catalysts, (v)understand the catalytic redox chemistry of Gly-Gly in thepresence of platinum group metal ions, (vi) find the catalyticefficiency of catalysts and (vii) compare the reactivity with thatunder uncatalysed oxidation.

8

EXPERIMENTAL

Materials

Chloramine-T (Merck) was purified by the method of Morriset al.[3] An aqueous solution of CAT was prepared, standardisediodometrically and stored in amber coloured, stoppered bottlesto prevent photochemical deterioration.[3] The concentration ofstock solutions was periodically determined. Chromatographi-cally pure Gly-Gly (Merck) was used as received. Aqueoussolutions of desired strength were prepared whenever required.Ruthenium trichloride (Merck), palladium chloride (Arora-Matthey) and platinum chloride (SD Fine Chem Ltd.) solutionswere prepared in 20mM HCl while osmium tetroxide (BDH) wasprepared in 20mMNaOH. Allowance for the amount of acid/alkalipresent in the catalyst solutions was made while preparing thereaction mixtures for kinetic runs. The ionic strength of thesystem was maintained constant at a high value (0.3mol dm�3)using a concentrated solution of NaClO4 in order to swamp thereaction wherever the effect is noticed. The permittivity of thereaction mixture was altered by the addition of methanol invarying proportions (v/v), and the values of the dielectricconstant of water–methanol mixtures as reported in theliterature[15] were employed. Solvent isotope studies were madein D2O (99.4% purity) supplied by Bhabha Atomic ResearchCenter, Mumbai, India. Reagent grade chemicals and doublydistilled water were used throughout. UV-3101 PC UV–Vis-NIRScanning Spectrophotometer was used for studying theformation of various complexes. Regression analysis of theexperimental data was carried out on an fx-100W scientificcalculator to obtain the regression coefficient, r.

Kinetic measurements

The reactions were carried out under pseudo-first-orderconditions ([substrate]0� [oxidant]0) at constant temperaturein glass-stoppered Pyrex boiling tubes coated black on outside

J. Phys. Org. Chem. 2008, 21 844–858 Copyright � 2008 John W

surface to eliminate any photochemical effects. The oxidant andrequisite amount of substrate, NaOH, Ru(III)/Os(VIII)/Pd(II)/Pt(IV)catalyst, NaClO4 (wherever mentioned) solutions and water (tokeep the total volume constant for all runs), were taken in thetube and thermostatted at 308 K until thermal equilibrium wasattained. A measured amount of CAT solution, which was alsothermostatted at the same temperature, was rapidly added to themixture and was shaken intermittently for uniform concentration.The progress of the reaction was monitored by iodometricdetermination of unreacted CAT in 5ml of aliquots of the reactionmixture withdrawn at different time intervals. The course ofthe reaction was studied for more than two half-lives. Thepseudo-first-order rate constants (k0 s�1) calculated from thelinear plots of log [CAT] versus time were reproducible within�3–5%.

Stoichiometry

Different ratios of CAT to Gly-Gly in the presence of1.0� 10�2mol dm�3 NaOH and 1.0� 10�5mol dm�3 catalystwere equilibrated at 308 K for 24 h. The unreacted oxidant in thereaction mixture was determined iodometrically. The analysisshowed that onemole of Gly-Gly reacts with twomoles of oxidantfor all the catalysed reactions, confirming to the followingstoichiometry:

NH2CH2CONHCH2COOHþ 2TsNClNaþ 3H2O ! 2HCHOþ 2CO2

þ 2NH3 þ 2TsNH2 þ 2Naþ þ 2Cl�

(1)

where Ts¼CH3C6H4SO2.

Product analysis

The reaction mixtures (1mol of Gly-Gly and 2moles of CAT in thepresence of 1.0� 10�2 and 1.0� 10�5mol dm�3 catalyst) wereallowed to progress for 2–3 h under stirred condition at 35 8C.After completion of the reaction (monitored by TLC), the reactionproducts were extracted with ether. Formaldehyde andp-toluenesulfonamide (PTS) are identified as the main oxidationproduct of Gly-Gly and reduction product of CAT, respectively.Evaporation of the ether layer was followed by columnchromatography on silica gel (60–200 mesh) using gradientelution (from dichloromethane to chloroform). After initialseparation, the products were further purified by recrystalisation.Formaldehyde, the oxidation product of Gly-Gly was detected

by spot tests[16] and estimated as its 2,4-DNP derivative.[17] Theamount of formaldehyde was calculated from the weight ofhydrazone formed in the presence of each catalyst and the yieldof the formaldehyde was found to be 75–80%. Further, themelting point of the 2,4-DNP derivative of formaldehyde wasfound to be 165 8C (Lit. m.p. 166 8C). It was also observed thatformaldehyde does not undergo further oxidation under theprevailing kinetic conditions.The reduction product of CAT, PTS (TsNH2), was detected by

TLC.[18] It was further confirmed by its melting point of 139 8C (Litm.p. 137–140 8C). Further, PTS has been quantitatively esti-mated[17] by its reaction with xanthydrol to yield the correspond-ing N-xanthyl-p-toluenesulfonamide. In a typical experiment,equal quantities of separated PTS and xanthydrol (0.20 g) weredissolved in 10ml of glacial acetic acid. The reaction mixture wasstirred for 3min at laboratory temperature and allowed to standfor 90min. The derivative was filtered, recrystalised with dioxane/

iley & Sons, Ltd. www.interscience.wiley.com/journal/poc

45

Table 1. Effect of varying reactant concentrations on the reaction rate at 308 K

103 [CAT]0 (mol dm�3) 102 [Gly-Gly]0 (mol dm�3)

104 k0(s�1)

Ru(III) Os(VIII) Pd(II) Pt(IV)

0.20 1.00 5.69 10.11 3.16 4.200.50 1.00 5.74 9.98 3.10 4.291.00 1.00 5.78 10.0 3.12 4.262.00 1.00 5.70 10.1 3.00 4.214.00 1.00 5.81 10.0 3.20 4.301.00 0.50 5.70 7.41 2.14 4.211.00 1.00 5.78 10.0 3.12 4.261.00 2.00 5.71 14.1 4.46 4.201.00 3.00 5.80 17.40 5.51 4.191.00 4.00 5.79 20.1 6.56 4.30

[NaOH]¼ 1.00� 10�2mol dm�3; [catalyst]¼ 1.00� 10�5mol dm�3; I¼ 0.30mol dm�3 (in case of Ru(III) catalysis).

R. V. JAGADEESH AND PUTTASWAMY

846

water (3:1) and dried at room temperature. This was weighed andthe recovery of PTS in all the cases is found to be 80–85%. Theliberated CO2 and NH3 were identified by conventional lime waterand Nessler’s reagent tests, respectively.

RESULTS

Kinetic orders

The kinetics of oxidation of Gly-Gly by CAT was investigated atseveral initial concentrations of the reactants in NaOHmedium inthe presence of Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysts at 308 Kunder identical experimental conditions. Despite the fact that thestoichiometry and oxidation products were same in all thecatalysed reactions, the oxidation kinetic behaviour was different.All the reactions were carried out under pseudo-first-orderconditions, wherein [Gly-Gly]0� [CAT]0.Under the pseudo-first-order conditions of [Gly-Gly]0� [CAT]0,

with other reaction conditions remaining constant, the order in[CAT]0 was found to be unity in all the four catalysed reactions, as

Table 2. Effect of varying NaOH and catalyst concentrations on t

102 [NaOH] (mol dm�3) 105 [catalyst] (mol dm�3)

0.20 1.000.50 1.001.00 1.002.00 1.004.00 1.001.00 0.201.00 0.501.00 1.001.00 2.001.00 4.00

[CAT]0¼ 1.00� 10�3mol dm�3; [Gly-Gly]0¼ 1.00� 10�2mol dm�3;

www.interscience.wiley.com/journal/poc Copyright � 2008

indicated by the linearity of the plots of log [CAT] versus time(r> 0.9948). Further, the linearity of these plots with constancy ofthe slopes obtained at various [CAT]0 indicated a first-orderdependence of the reaction rate on [CAT]0. The pseudo-first-order rate constants (k

0) obtained are given in Table 1.

Under the same experimental conditions, the rate of the reactionincreased in [Gly-Gly]0 (Table 1) for Os(VIII)- and Pd(II)-catalysedreactions and plots of log k

0versus log [Gly-Gly] were found to be

linear (r> 0.9925) with fractional slopes of 0.47 and 0.54,respectively, indicating a fractional-order dependence on[Gly-Gly]0 in both cases. Further, the intercepts of the plots ofk0versus [Gly-Gly]0 in these cases were linear (r> 0.9889) which

confirmed a fractional-order dependence of rate on [Gly-Gly]0 inboth the reactions. But the orders in [Gly-Gly]0 were found to bezero in case of Ru(III) and Pt(IV) catalysis (Table 1).The rate of the reaction increased with increasing [NaOH]

(Table 2) in all the cases. The log–log plots of rate versus [NaOH](r> 0.9901) showed that the orders in [OH�] were less than unity(0.37–0.73), indicating a fractional-order dependence of rate on[OH�] in all the four catalysed reactions. The reaction rateincreases with the increase in [Ru(III)] and [Pd(II)] and log–log

he reaction rate at 308 K

104 k0(s�1)


2.41 5.42 1.06 2.264.00 7.61 2.01 3.175.76 10.0 3.12 4.268.41 13.6 5.76 5.47

12.2. 17.7 9.01 7.061.14 3.00 0.74 1.162.86 6.10 1.47 2.515.76 10.00 3.12 4.26

11.6 17.7 6.41 7.6723.2 29.6 12.61 14.0

I¼ 0.30mol dm�3 (in case of Ru(III) catalysis).

John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2008, 21 844–858

Figure 1. Plot of log k0versus I1/2

Figure 2. Plots of log k0versus 1/D


plots of rate versus [catalyst] were found to be linear (r> 0.9905)with a slope of unity, indicating a first-order dependence of rateon [Ru(III)] and [Pd(II)]. Whereas in case of [Os(VIII)] and [Pt(IV)]catalysis, such plots were linear (r> 0.9965) with slopes less thanunity (0.80–0.85), indicating fractional-order kinetics in [Os(VIII)]and [Pt(IV)]. All these results are recorded in Table 2.Addition of PTS to the reactionmixture retards the rate in Pt(IV)

catalysis and the rate constants at 4.0, 8.0, 14.0 and 20.0� 10�4

[PTS]mol dm�3 were 4.26, 3.80, 2.92 and 2.45� 10�4 s�1,respectively. Further, a plot of log k

0versus log [PTS] was linear

(r¼ 0.9899) with a negative slope of 0.25, indicating a negativefractional-order dependence of the rate on [PTS]. It also indicatesthat PTS is involved in a fast pre-equilibrium to the rate-limitingstep in the proposed scheme for the Pt(IV) catalysis. But the ratewas unaffected by the addition of PTS in other catalysed reactionsand it signifies that the PTS is not involved in any step prior to therate-limiting step in the schemes proposed.An increase in ionic strength of a reaction system by the

addition of NaClO4 decreased the rate of the reaction and the rateconstants at 0.1, 0.2, 0.3, 0.4 and 0.5mol dm�3 ionic strength were12.8, 8.11, 5.76, 4.21 and 3.11� 10�4 s�1, respectively, in case ofRu(III) catalysis. Further, a plot of log k

0versus I1/2 gave a straight

line (Fig. 1; r¼ 0.9926) with a negative slope of 1.5. But variation ofionic strength showed negligible effect on the rate of the reaction

Table 3. Effect of varying dielectric constant of the medium on t

% MeOH (v/v) D Ru(III)

0 76.73 5.7610 72.37 6.8120 67.48 9.0030 62.71 12.01

[CAT]0¼ 1.00� 10�3moldm�3; [Gly-Gly]0¼ 1.00� 10�2mol dm�3; [NI¼ 0.30mol dm�3 (in case of Ru(III) catalysis).


in other catalysed reactions. Hence, only in case of Ru(III)-catalysed reaction, the ionic strength of the medium wasmaintained at a constant concentration of 0.30mol dm�3 ofNaClO4 for kinetic runs in order to swamp the reaction.The effect of dielectric constant of the medium on the reaction

rate was studied by adding methanol (0–30% v/v) to the reactionmixture, while keeping other experimental conditions constant.The rates were found to increase with increase in MeOH contentin case of Ru(III)- and Os(VIII)-catalysed reactions and the plots oflog k

0versus 1/D were linear (Fig. 2; r> 0.9979) with positive

slopes. But in the case of Pd(II) catalysis, the slope of such a plotwas negative (Fig. 2; r¼ 0.9990). However, negligible influence ofvariation of dielectric constant on the rate was observed in caseof Pt(IV) catalysis. All these results were recorded in Table 3.The rate remained constant with the addition of Cl� or Br� ions in

he reaction rate at 308 K

104 k0(s�1)

Os(VIII) Pd(II) Pt(IV)

10.0 3.12 4.2612.3 2.41 4.3015.2 1.61 4.2020.0 1.00 4.16

aOH]¼ 1.00� 10�2mol dm�3; [catalyst]¼ 1.00� 10�5moldm�3;


847

Table 4. Temperature dependence on the reaction rate and values of activation parameters for the oxidation of Gly-Gly by CAT inthe presence and absence of catalyst

Temperature (K)

104 k0(s�1)


298 2.84 5.69 1.20 1.79 (0.11)303 3.74 7.82 2.01 2.71 (0.22)308 5.76 10.0 3.12 4.26 (0.41)313 8.08 14.0 5.11 6.61 (0.81)318 11.6 18.2 8.00 9.59 (1.41)Ea (kJmol�1) 58.6 42.7 75.7 68.2 (108)DH 6¼ (kJmol�1) 56.0� 0.02 39.6� 0.01 73.1� 0.02 65.3� 0.06 (106� 0.10)DG6¼ (kJmol�1) 95.0� 0.09 93.1� 0.16 95.8� 0.2 95.0� 0.12 (101� 0.21)DS 6¼ (J K�1mol�1) �125� 0.21 �173� 0.21 �75.3� 0.10 �96.2��0.21 (15.7� 0.41)log A 9.69� 0.11 4.17� 0.01 9.33� 0.10 8.20� 0.01 (14� 0.09)

[CAT]0¼ 1.00� 10�3mol dm�3; [Gly-Gly]0¼ 1.00� 10�2mol dm�3; [NaOH]¼ 1.00� 10�2mol dm�3; [catalyst]¼ 1.00� 10�5mol dm�3;I¼ 0.30mol dm�3 (in case of Ru(III) catalysis).Values in the parentheses refer to oxidation of Gly-Gly by CAT in the absence of catalyst. Experimental conditions are the same as infootnote without catalyst.


848

the form of NaCl or NaBr (1.0� 10�2–5.0� 10�2mol dm�3). As adependence of the rate on hydroxyl ion concentration was noted,solvent isotope studies were made using D2O for all the fourcatalysed reactions. Studies of the reaction rate in D2O mediumfor Ru(III)-, Os(VIII)-, Pd(II)- and Pt(IV)-catalysed reactions revealedthat k

0(H2O)was equal to 5.70� 10�4, 10.0� 10�4, 3.12� 10�4

and 4.26� 10�4 s�1, and k0(D2O) was 6.7.0� 10�4, 11.2� 10�4,

3.85� 10�4 and 4.85� 10�4 s�1, respectively. The solvent isotopeeffect, k

0(H2O)/k

0(D2O) was found to be 0.88, 0.89, 0.81 and 0.88 for

the four cases.The reaction was studied at different temperatures (298–

318 K), keeping other experimental conditions constant. From thelinear Arrhenius plots of log k

0versus 1/T (r> 0.9904), values of

activation parameters for the overall reaction were evaluated(Table 4). The addition of the reaction mixtures to aqueousacrylamide monomer solutions did not initiate polymerisation,indicating the absence of in situ formation of free radical speciesin the reaction sequence. The control experiments were alsoperformed under the same reaction conditions but without theoxidant, CAT.

DISCUSSION

Chloramine-T acts as an oxidising agent in both acidic andalkaline solutions. The oxidation potential of CAT-PTS redoxcouple varies[19] with the pH of the medium, having values of1.139 V at pH 0.65, 0.778 V at pH 7.0 and 0.614 V at pH 9.7.Chloramine-T behaves like a strong electrolyte[2] in aqueoussolutions, and depending on the pH of the medium, it furnishesdifferent equilibria in solutions.[2,19–21] The possible oxidisingspecies in acidified CAT solutions[2,19–21] are the conjugate-freeacid TsNHCl, the dichloramine-T TsNCl2, the hypochlorous acidHOCl and also perhaps H2O

þCl. In alkaline CAT solutions, TsNCl2does not exist[22,23] and hence the expected reactive species areTsNHCl, TsNCl�, HOCl and OCl�.


Hardy and Johnston[19] have reported the establishment of thefollowing equilibria in alkaline solution of CAT:

TsNCl� þ H2O Ð TsNHClþ OH� (2)

TsNHClþ H2O Ð TsNH2 þ HOCl (3)

If HOCl were to be the primary oxidising species as indicated inEqn (3), a first-order retardation of rate by added PTS would beexpected. However, no such effect was noticed in the presentstudy. If TsNHCl is the reactive species, retardation of the rate by[OH�] is expected (Eqn (2)), which is also contrary to theexperimental observations. Hence, in the present investigation,the rate of the reaction is accelerated by OH� ions clearlyindicates that the anion TsNCl� is themost likely oxidising speciesinvolved in the oxidation of Gly-Gly by CAT in all the fourcatalysed reactions.

Mechanism and rate law of Ru(III) catalysis

Ru(III) chloride acts as an efficient catalyst in many redoxreactions, particularly in an alkaline medium.[24–26] Under theexperimental conditions [OH�]� [Ru(III)] and the fact that [OH�]increases the rate, ruthenium (III) is mostly present as thehydroxylated species [Ru(H2O)5OH]

2þ and its formation in thefollowing equilibrium is of importance in the reaction:

½RuðH2OÞ6�3þ þ OH� Ð ½RuðH2OÞ5OH�

2þ þ H2O (4)

Hence, the hydroxylated species [Ru(H2O)5OH]2þ complex ion

of ruthenium(III) has been assumed to be the reactive catalysingspecies. Similar equilibria have been reported between Ru(III)-catalysed oxidation of several other substrates with variousoxidants in alkaline medium.[13,27–29]

The existence of a complex between the catalyst and oxidantwas evidenced from the UV–Vis spectra of both Ru(III) and



Ru(III)-CATmixture, in which a shift of Ru(III) from 352.6 to 338 nmwas observed, indicating the formation of a complex (Fig. 3). Suchtype of oxidant–catalyst complex formation has also beenreported in other studies.[30,31]

Further, for a general equilibrium

Mþ nS ÐKðMSnÞ (5)

between two metal species, M and MSn having differentextinction coefficients, Ardon[32] has derived the following:

1

DA¼ 1

½S�nf1=DE½MTotal�Kgþ 1

DE½MTotal�(6)

where K is the formation constant of the complex, [S] is theconcentration of CAT, DE is the difference in extinction coefficientbetween two metal species, [M]Total is the total concentration ofmetal species and DA is the absorbance difference of solutioncontaining no S and one that contains a certain concentration ofS represented by [S]. Equation (6) is valid provided that [S] is somuch greater than [M]Total that the amount of S tied up in thecomplex is negligible or it is subtracted from the initialconcentration of S. According to Eqn (6), a plot of 1/DA versus1/[S] or 1/[S]2 should be linear with an intercept in case of 1:1 or1:2 type of complex formation between M and S. The ratio ofintercept to slope of this linear plot gives the value of K.Ruthenium (III) in NaOH medium containing CAT showed an

absorption peak at 338 nm (lmax for the complex). The complexformation studies were made at this lmax of 338 nm. In a setof experiments, the solutions were prepared by takingdifferent amounts of CAT (2.0� 10�4–4.0� 10�3mol dm�3) atconstant amounts of RuCl3 (1.0� 10�5mol dm�3) and NaOH(1.0� 10�2mol dm�3) at 308 K. The absorbance of thesesolutions was measured at 338 nm. The absorbance of thesolution in the absence of CAT was also measured atthe same wavelength. The difference of these absorbances (withand without CAT) gives the differential absorbance, DA. A plot of1/DA versus 1/[CAT] was linear (r¼ 0.9944) with an interceptsuggesting the formation of a 1:1 complex between CAT andRu(III) catalyst. Similar type of behaviour for the formation ofcomplex has been reported in earlier works.[18,32,33] Further, the

Figure 3. UV–Vis spectra of (A) CAT, (B) Ru(III) and (C) CATþ Ru(III) in NaOH


plot of log (1/DA) versus log (1/[CAT]) was also linear (r¼ 0.9801)and from the slope and intercept of the plot of 1/DA versus 1/[CAT], the value of the formation constant, K, of the complex wasdeduced; it was found to be 6.06� 102.In view of the above facts, a detailed mechanism for

Ru(III)-catalysed oxidation of Gly-Gly by CAT in alkaline mediumis shown in Scheme 1.The total concentration of CAT is [CAT]t, then

½CAT�t ¼ ½TsNHCl� þ ½TsNCl�� (7)

By substituting for [TsNHCl] from equilibrium (i) of Scheme 1 inEqn (7) and solving for [TsNCl�], one obtains

½TsNCL�� ¼ K1½CAT�t½OH��½H2O�þK1½OH�� (8)

From the slow and rate-determining step (rds) of Scheme 1,

Rate ¼ �d½CAT�t ¼ k2½TsNCl��½RuðIIIÞ� (9)

By substituting for [TsNCl�] from Eqn (8) into Eqn (9), thefollowing rate law is obtained:

Rate ¼ �d½CAT�tdt

¼ K1K2½CAT�t½RuðIIIÞ�½OH��½H2O� þ K1½OH�� (10)

Rate law (10) is in good agreement with the experimentalresults.Since rate¼ k

0[CAT]t, Eqn (10) can be transformed into

k0 ¼ K1k2½RuðIIIÞ�½OH��½H2O� þ K1½OH�� (11)

1

k0¼ ½H2O�

K1k2½RuðIIIÞ�½OH�� þ1

k2½RuðIIIÞ�(12)

Based on Eqn (12), a plot of 1/k0versus 1 /[OH�] at constant

[CAT]0, [Gly-Gly]0, [Ru(III)] and temperature was found to belinear (r¼ 0.9822). From the values of slope and intercept of sucha plot, the equilibrium constant (K1) and decompositionconstant (k2) were calculated and found to be 7.90� 104 and11.1 dm3mol�1 s�1, respectively.

medium


849

Scheme 1. A detailed mechanistic scheme for Ru(III)-catalysed oxidation of Gly-Gly by CAT


850

The proposed mechanism and the derived rate law aresubstantiated by the following experimental facts:For a reaction involving a fast pre-equilibrium Hþ or OH� ion

transfer, the rate increases in D2O since D3Oþ and OD� are two to

three times stronger acids and stronger bases,[34–36] respectively,than H3O

þ and OH� ions. In the present study, the observedsolvent isotope effect of k

0(H2O)/k

0(D2O)< 1 is due to the greater

basicity of OD� compared to OH� and is conforming to the fact.However, the magnitude of increase of rate in D2O is small(k

0(H2O)/k

0(D2O)¼ 0.88) compared to the expected value of 2 to 3

times greater, which can be attributed to the fractional-orderdependence of rate on [OH�].The ionic strength (I) effect on the reaction rates has been

described according to the theory of Bronsted and Bjerrum,[37]

which postulates the reaction through the formation of anactivated complex. According to this theory, the effect of ionic


strength on the rate for a reaction involving two ions is given bythe relationship:

log k0 ¼ log k0 þ 1:02ZAZBI1=2 (13)

Here, ZA and ZB are the valency of the ions A and B, and k and k0are the rate constants in the presence and absence of the addedelectrolyte, respectively. A plot of log k

0against I1/2 should be

linear with a slope of 1.02 ZAZB. If ZA and ZB have similar signs, thequantity ZAZB is positive and the rate increases with the ionicstrength, having a positive slope, while if the ions have dissimilarcharges, the quantity ZAZB is negative and the rate woulddecrease with the increase in ionic strength, having a negativeslope. In the present case, a primary salt effect is observed as therate decreases with increase in ionic strength of medium,[37]

supporting the involvement of ions of opposite sign in the



rate-limiting step (Scheme 1). The Debye–Huckel plot of log k0

against I1/2 gave a straight line with a slope of�1.5. In the presentsystem, two positive ions and one negative ion are involved in therate-limiting step (Scheme 1) and the expected slope of �2 hasnot been found. This may be due to the fact that the ionicstrength employed in the present investigation is beyond theformal Debye–Huckel limiting law. Alternatively, there could beformation of ion pairs in concentrated solutions, as suggested byBjerrum.[37]

In the present case, addition of methanol to the reactionmixture increased the reaction rate. The effect of changingsolvent composition on the rate of reaction has been described indetail in various monographs[38–44] and a plot of log k

0versus 1/D

was linear, with a positive slope. The dependence of the rateconstant on the dielectric constant of the medium is given by thefollowing:

ln k0 ¼ ln k00 �NZAZBe

2

DRTr 6¼

� �(14)

In this equation, k00 is the rate constant in a medium of infinitedielectric constant, ZAe and ZBe are the total charges on the ions Aand B, r6¼ is the radius of the activated complex, R, T and N havetheir usual meanings. This equation predicts a linear plot of log k

0

versus 1/D with a negative slope if the charges on the ions are ofthe same sign and a positive slope if they are of opposite sign. Thepositive dielectric effect observed in the present study (Table 3)clearly supports[41] the involvement of dissimilar charges in therate-limiting step (Scheme 1).It was felt reasonable to compare the reactivity of CAT towards

Gly-Gly in the absence of Ru(III) catalyst, under identicalexperimental conditions, in order to evaluate the catalyticefficiency of Ru(III). The reactions were carried out at differenttemperatures (298–318 K) and from the plots of log k

0versus 1/T

(r> 0.9914), activation parameters are evaluated for theuncatalysed reactions also (Table 4). However, the Ru(III)-catalysed reactions were found to be about 14 times faster thanuncatalysed reactions. The activation parameters evaluated forthe catalysed and uncatalysed reactions explain the catalyticeffect on the reaction. The catalyst Ru(III) forms a complex (X) withthe oxidant, which increases the oxidising property of the oxidant

Table 5. Values of catalytic constant (KC) at different temperature

Temperature (K) Ru(III) Os(V

298 27.3 5.5303 35.2 7.5308 53.5 9.5313 72.7 13318 102 16Ea (kJmol�1) 51.0 39DH 6¼ (kJmol�1) 48.4� 0.01 36.7�DG6¼ (kJmol�1) 65.5� 0.16 69.7�DS 6¼ (J K�1mol�1) �54.6� 0.13 �106�log A 10.3� 0.10 7.66�

[CAT]0¼ 1.00� 10�3moldm�3; [Gly-Gly]0¼ 1.00� 10�2mol dm�3; [NI¼ 0.30mol dm�3 (in case of Ru(III) catalysis).


than without Ru(III). Further, the catalyst Ru(III) suitably modifiesthe reaction path by lowering the energy of activation (Table 4).It has been pointed out by Moelwyn-Hughes[45] that, even in

presence of the catalyst, the uncatalysed reactions also proceedsimultaneously, so that

k1 ¼ k0 þ KC½catalyst�x (15)

Here, k1 is the observed pseudo-first-order rate constantobtained in the presence of Ru(III) catalyst and k0 is that for theuncatalysed reaction, KC is the catalytic constant and x is the orderof the reaction with respect to [Ru(III)]. In the presentinvestigation, ‘x’ value was found to be unity. Then the valueof KC is calculated using the Eqn (16):

KC ¼ k1 �k0

½RuðIIIÞ�x (16)

The values of KC have been evaluated at different temperatures(298–318 K) and KC was found to vary with temperature. Further, aplot of log KC versus 1/T was linear (r¼ 0.9992) and the values ofenergy of activation and other activation parameters for theRu(III) catalyst were computed and are summarised in Table 5.The proposed mechanism is supported by the observed

moderate values of energy of activation and other thermodyn-amic parameters. The fairly high positive values of the free energyof activation and of the enthalpy of activation suggest that thetransition state is highly solvated, while fairly high negativeentropy of activation (Table 4) indicates the formation of a rigidassociated transition state. Addition of the reduction product ofCAT, PTS, did not alter the rate, indicating its non-involvement inthe pre-equilibrium with the oxidant. Similarly, addition of halideions has no effect on the rate indicating that there is no role forhalide ions in the reaction. All these observations support theproposed mechanism.

Mechanism and rate law of Os (VIII) catalysis

Osmium tetroxide is known to be an efficient catalyst in theoxidation of several organic compounds by various oxidants inaqueous alkaline medium.[29,46–48] It has been shown thatosmium is stable in its þ8 oxidation state and exists in the

s and activation parameters calculated using KC values

KC

III) Pd(II) Pt(IV)

7 0.61 2.989 1.00 4.428 1.52 6.84.2 2.41 10.4.8 3.70 14.5.2 68.2 59.80.06 65.5� 0.01 57� 0.10.14 74.5� 0.12 70.3� 0.240.14 �28.9� 0.10 �43.0��0.610.01 11.7� 0.11 10.6� 0.10

aOH]¼ 1.00� 10�2mol dm�3; [catalyst]¼ 1.00� 10�5moldm�3;


851

Figure 4. UV–Vis Spectra of (A) CAT, (B) Gly-Gly and (C) CATþGly-Gly, (D) Pd(II) and (E) Pd(II)þCATþGly-Gly in NaOH medium


852

following equilibria:[49–55]

OsO4 þ OH� þ H2O Ð ½OsO4ðOHÞðH2O�� (17)

½OsO4ðOHÞðH2OÞ�� þ OH� Ð ½OsO4ðOHÞ2�2� þ H2O (18)

The complexes [OsO4(OH)(H2O)]� and [OsO4(OH)2]

2�, whichcan be reduced to [OsO2(OH)4]

2�, with octahedral geometriesare less likely to form higher coordination species with theoxidant/substrate. It is more realistic to postulate OsO4, which hastetrahedral geometry,[50] as the active catalyst species than caneffectively form a complex with the oxidant/substrate species.Spectroscopic evidence for the complex formation betweenCATand Gly-Gly was obtained from UV–Vis spectra of Gly-Gly, CATand a mixture of both (Fig. 4). Absorption maxima in alkalinemedium appear at 236 nm for Gly-Gly, 223 nm for CATand 227 nmfor a mixture of both. A bathochromic shift of 4 nm from 223 to227 nm of CAT suggests that complexation occurs between CATand Gly-Gly.Furthermore, the first-order dependence of rate on [CAT]0 and

fractional-order dependence on each of [Gly-Gly]0, [OH�] and

[Os(VIII)] indicate that the intermediate complex formed from theoxidant and substrate, which interacts with the catalyst. Thepossible oxidising species here would be TsNCl�. In the light ofthese considerations, the Gly-Gly-CAT oxidation mechanism isformulated as given in Scheme 2 to explain all the observedexperimental results.The total effective concentration of CAT is

½CAT�t ¼ ½TsNHCl� þ ½TsNCl�� þ ½XII� þ ½XIII� (19)

By substituting for [TsNHCl], [TsNCl�] and [XII] from steps (i), (ii)and (iii) of Scheme 2 in Eqn (19) and solving for [XIII], we get

½XIII� ¼ K5K6K7½CAT�t½Gly� Gly�½OsðVIIIÞ�½OH��½H2O� þ K5½OH�� þ K5K6½Gly� Gly�½OH��f1þ K7½OsðVIIIÞ�g

(20)

From the slow and rds of Scheme 2,


¼ k8½XIII� (21)


By substituting for [XIII] from Eqn (20) into Eqn (21), thefollowing rate law was obtained:

Rate ¼ K5K6K7k8½CAT�t½Gly� Gly�½OsðVIIIÞ�½OH��½H2O� þ K5½OH�� þ K5K6½Gly� Gly�½OH��f1þ K7½OsðVIIIÞ�g

(22)

Since rate¼ k0[CAT]t, Eqn (22) can be transformed into

k0 ¼ K5K6K7K8½Gly� Gly�½OsðVIIIÞ�½OH��½H2O� þ K5½OH�� þ K5K6½Gly� Gly�½OH��f1þ K7½OsðVIIIÞ�g

(23)

Scheme 2 and the rate law (23) are supported by the followingfacts:Solvent isotope studies show that k

0(H2O)/k

0(H2O)< 1. This is

generally correlated with the fact that OD� ion is a stronger basethan OH� and in base catalysed reactions, enhancement of ratein D2O medium is expected[34–36] and the same is noticed in thepresent study. In the present investigation, a plot of log k

0versus

1/D is linear with a positive slope supporting the proposedScheme 2, where a positive ion and a dipole[43,56] are involved inthe rate-limiting step. The negligible influence of variation of theionic strength, addition of PTS and halide ions on the rate of thereaction, and also the evaluated activation parameters are ingood agreement with the mechanism proposed and the rate lawderived.The reactivity of CAT towards Gly-Gly in the absence of Os(VIII)

catalyst was compared with the Os(VIII)-catalysed reaction, underidentical set of experimental conditions. Rate constants revealedthat the Os(VIII)-catalysed reactions are 24-fold faster thanuncatalysed reactions (Table 4). The values of KC were determinedat different temperatures and from a plot of log KC versus 1/T(r¼ 0.9948), values of activation parameters for Os(VIII) catalystwere computed (Table 5).

Mechanism and rate law of Pd(II) catalysis

Palladium (II) chloride catalysis has been observed during variousredox reactions and the reactions in the presence of Pd(II)have also shown a complex kinetics. Generally, the palladium


Scheme 2. A detailed mechanistic scheme for Os(VIII)-catalysed oxidation of Gly-Gly by CAT


8

complexes are somewhat less stable, from both kinetic andthermodynamic sense, than their platinum analogous. It is knownto exist as different complexes in alkaline solutions[13,57–59]

and the possible Pd(II) complex species are [Pd(OH)Cl3]2�,

[Pd(OH)2Cl2]2� and [Pd(OH)4]

2�. The species [Pd(OH)3Cl]2� and

[Pd(OH)4]2� are not commonly found as they are insoluble.

Further, the rate increases with the increase in [OH�] and therewas no effect of [Cl�] on the rate of reaction which clearly rulesout [Pd(OH)Cl3]

2� as the reactive species. Hence, [Pd(OH)2Cl2]2�

complex ion has been assumed to be the reactive species in thepresent study.UV–Vis spectral studies revealed that there is transient

existence of complex between CAT and Gly-Gly in alkaline


medium for Pd(II) catalysis. Absorption maxima were appearedat 223 nm for CAT, 236 nm for Gly-Gly and 227 nm for mixtureof both (Fig. 4). The formation of other complex betweenCATþGly-Gly mixture and Pd(II) was also informed from UV–Visspectral studies. Absorption maxima in alkaline mediumappeared at 356 nm for Pd(II), 227 nm for CATþGly-Gly and340 nm for Pd(II) and CATþGly-Gly mixture confirmative theexistence of complex between Pd(II) and CATþGly-Gly mixture.The complex formation studies have been made for Pd(II) andCATþGly-Gly mixture at 340 nm in alkaline medium. A plot of 1/DA versus 1/[CAT] (r¼ 0.9910) with an intercept suggests theformation of 1:1 complex between Pd(II) and CATþGly-Glymixture. Further, the 1:1 complex formation was also evidenced


53

Scheme 3. A detailed mechanistic scheme for Pd(II)-catalysed oxidation of Gly-Gly by CAT


854

from the linearity of the plot log 1/DA versus log 1/[CAT](r¼ 9899). From the slope and intercept of the plot 1/DA versus 1/[CAT], the value of formation constant, K was found to be3.5� 102.Furthermore, the kinetic data led the authors to assure that

TsNCl� is the reactive species of CAT. On the basis of theexperimental findings, Scheme 3 is proposed for CAToxidation ofGly-Gly in the presence of Pd(II) in alkaline medium.The total effective concentration of CAT is

½CAT�t ¼ ½TsNHCl� þ ½TsNCl�� þ ½XV� (24)

By substituting for [TsNHCl] and [TsNCl�] from steps (i) and (ii)of Scheme 3 in Eqn (24) and solving for [XV], one gets

Rate ¼ K10K11K12½CAT�t½Gly� Gly�½OH��½H2O� þ K10½OH�� þ K10K11½Gly� Gly�½OH�� (25)


From the slow and rds of Scheme 3,


¼ k12½XV�½PdðIIÞ� (26)

By substituting for [XV] from Eqn (25) into Eqn (26), thefollowing rate law is derived:

Rate ¼ K10K11K12½CAT�t½Gly� Gly�½PdðIIÞ�½OH��½H2O� þ K10½OH�� þ K10K11½Gly� Gly�½OH�� (27)

The rate law (27) satisfies all the experimental observations andhence the proposed mechanism and the derived rate law arevalid.Since rate¼ k

0[CAT]t, Eqn (27) can be transformed into

k0 ¼ K10K11K12½Gly� Gly�½PdðIIÞ�½OH��½H2O� þ K10½OH�� þ K10K11½Gly� Gly�½OH�� (28)


Figure 5. UV–Vis spectra of (A) CAT, (B) Pt(IV) and (C) Pt(IV)þCAT in NaOH medium


Rate of reaction decreases on decreasing dielectric constant ofthe medium. This observed solvent effect leads to the reportedconclusion,[43] that the decrease in dielectric constant shoulddecrease the reaction rate for the reaction involving a negativeion and dipolar molecule as shown in Scheme 3, whichsubstantiates the proposed mechanism. The increase of reactionrate in D2Omedium supports the proposed mechanism since it iswell known that OD� is a stronger base than OH�[34–36] ion andhence exerts a stronger acceleration effect on the reaction. Theproposed mechanism is also supported by the activationparameters and also negligible effect of PTS, halide ions andionic strength on the rate of the reaction. Further, energy ofactivation of Pd(II)-catalysed and -uncatalysed reactions werecomputed and it was found that Pd(II)-catalysed reactions werenearly eightfold faster than uncatalysed reactions (Table 4). Thevalues of KC and activation parameters for Pd(II) catalyst weredetermined (Table 5) by the plot of log KC versus 1/T (r¼ 0.9981).

8

Mechanism and rate law of Pt(IV) catalysis

Pt(IV) catalysis during variety of redox-reactions is well reportedin the literature.[13,33,60,61] Chloroplatinic acid, H2(PtCl6), is thestarting material in platinum(IV) chemistry. In aqueous solution,chloroplatinic acid ionises[13,33,62] as follows:

H2½PtCl6� Ð ½PtCl6�2� þ 2Hþ (29)

In an alkaline medium (pH> 8), [PtCl6]2� changes to [PtCl5

(OH)]2� in a fast step[33,60,62] and follows as

½PtCl6�2� þ OH� Ð ½PtCl5ðOHÞ�2� þ Cl� (30)

Further ligand (Cl�) replacements from [PtCl5 (OH)]2� is alsoreported:[33,60,62]

½PtCl5ðOHÞ�2� þ OH� Ð ½PtCl4ðOHÞ2�2� þ Cl� (31)

However, the dihydroxy platinum (IV) species is quiteunstable[33,63] in aqueous solutions and therefore consequentlyunder the present experimental conditions [PtCl5(OH)]

2�may actas the reactive species of Pt(IV) in alkaline medium. Similarequilibrium have been reported in the Pt(IV)-catalysed oxidation


of several substrates using various oxidants in alkaline med-ium.[60–62]

The formation of a complex between Pt(IV) and oxidant wasevidenced from UV–Vis spectra of both Pt(IV), and Pt(IV)-CAT inwhich a shift of Pt(IV) from 360 to 347 nm was observed,indicating the formation of a complex (Fig. 5). According toEqn (6), a plot of 1/DA versus 1/[CAT] (r¼ 0.9912) with an interceptsuggests the formation of 1:1 complex between Pt(IV) and CAT.Further, the plot of log 1/DA versus log 1/[CAT] was also linear(r¼ 9816). From the slope and intercept of the plot 1/DA versus 1/[CAT], the value of formation constant, K, of the complex wasfound to be 4.29� 102.Based on the experimental results, it is likely that TsNCl� itself

acts as the reactive oxidant species in the present case also.Considering the above facts and all the observed experimentaldata, the reaction Scheme 4 can be suggested for Pt(IV)-catalysedoxidation of Gly-Gly by CAT in alkaline medium.The total effective concentration of CAT is

½CAT�t ¼ ½TsNHCl� þ ½TsNCl�� þ ½XVII� þ ½XVIII� (32)

By substituting for [TsNHCl], [TsNCl�] and [XVII] from steps (i), (ii)and (iii) of Scheme 4 in Eqn (32) and solving for [XVIII], we get

½XVIII� ¼ K14K15K16½CAT�t½PtðIVÞ�½OH��½H2O�½TsNH2�f½H2O� þ K14½OH�� þ K14K15½PtðIVÞ�½OH��g þ 1

(33)

From the slow and rds (iv) of Scheme 4,


¼ k17½XVIII� (34)

By substituting for [XVIII] from Eqn (33) into Eqn (34), thefollowing rate law is governed:

Rate ¼ K14K15K16k17½CAT�t½PtðIVÞ�½OH��½H2O�½TsNH2�f½H2O� þ K14½OH�� þ K14K15½PtðIVÞ�½OH��g þ 1

(35)

The rate law (35) is in complete agreement with theexperimental observations.


55

Scheme 4. A detailed mechanistic scheme for Pt(IV)-catalysed oxidation of Gly-Gly by CAT


856

Since rate¼ k0[CAT]t, Eqn (35) can be transformed into Eqn (36):

k0 ¼ K14K15K16k17½CAT�t½PtðIVÞ�½OH��½H2O�½TsNH2�f½H2O� þ K14½OH�� þ K14K15½PtðIVÞ�½OH��g þ 1

(36)

The observed dielectric and isotope effects are in accordancewith the theories of Amis[43] and Collins and Bowman,[34]

respectively. The negligible influence of ionic strength of themedium and of added halide ions on the rate of the reaction, andalso the observed activation parameters further corroborate withthe suggested mechanism. Catalytic constants and activationparameters with reference to Pt(IV) catalyst have been computed


(Table 5). Pt(IV)-catalysed reactions showed 10-fold faster thanthe uncatalysed reactions (Table 4). The values of k0 obtainedfrom the experiment (0.41� 10�4 s�1) and by the plot of k

0versus

Pt(IV) (0.39� 10�4 s�1) were consistent with each other,indicating that both Pt(IV)-catalysed and -uncatalysed reactionstake place simultaneously.We thought, it is worthwhile to compare the rate of oxidation

of Gly-Gly in the presence of these four catalysts with that ofuncatalysed reaction (in the absence of catalyst), under identicalexperimental conditions. It was found that the catalysedreactions are 7- to 24-fold faster than the uncatalysed reaction.For the catalysed reactions, it is seen from the Table 4 that theactivation energy is highest for the slowest reaction and vice



versa. From the inspection of rate constants and the values ofenergy of activation (Table 4), the relative reactivity of thesecatalysts during the oxidation of Gly-Gly by CAT in alkalinemedium is in the order: Os(VIII)> Ru(III)> Pt(IV)> Pd(II). This maybe attributed to the d-electronic configuration of the metal ions.Osmium having d0 electronic configuration has greater catalyticefficiency to oxidise the substrate compared to the other metalions used in the present study. Thus, the catalytic efficiencydecreases as the number of electrons in the d-orbital increases.Pd(II) having d8 electronic configuration is expected to have leastcatalytic efficiency among the catalysts used. It is likely thatduring the course of the reaction the metal ion momentarilyundergo reduction when the oxidant/oxidant-substrate complexis attached to themetal ions and after this themetal ion gets backto its original valence state as shown in Schemes 1–4. Ru(III) andPt(IV) having d5 and d6 electronic configuration, respectively,exhibit intermediate catalytic efficiency in the present study.Hence, based on d-electronic configuration of the metal ions, thereactivity decreases as the number of electrons increases in thed-orbital as d0 (Os(VIII))> d5 (Ru(III))>d6 Pt(IV)>d8 (Pd(II)).Consequently, the observed catalytic trend: Os(VIII)> Ru(III)>Pt(IV)> Pd(II) is based on the d-electronic configuration of themetal ions.Furthermore, efforts were made to compare the main salient

features of the kinetic data of this paper with the resultsreported[64] for the oxidation of glycine (monomer of gly-gly) byCAT in alkalinemedium. Gowda andMahadevappa have reportedthe kinetics of oxidation of several amino acids by CAT in bothacid and alkaline media.[64] Rate of oxidation of glycine showsfirst-order dependence each in [CAT]0 and [Glyine]0, and inversefractional-order in [OH�]. But in the present investigation, theoxidation of gly-gly exhibits a first-order kinetics in [CAT]0 andfractional-order in [OH�] for all the four catalysed reactions,whereas zero and fractional-order in [gly-gly]0, and first andfractional order in [catalyst] for various catalysed reactionsstudied. The stoichiometry of the reaction was found to be 1:2 forboth glycine and gly-gly. It was also found that the Ea¼ 108 and62.0 kJmol�1 for gly-gly (Table 4) and glycine,[64] respectively,revealed that the reaction is faster in glycine compared to gly-gly.This can be attributed to the involvement of different oxidisingspecies TsNHCl and HOCl in the oxidation of glycine, and TsNCl�

in the case of gly-gly. The facts presented above compel us toconclude that the present study in many respects differ from thereported studies[64] that is the kinetics of oxidation of glycine byCAT in alkaline medium.

8

CONCLUSIONS

The kinetic patterns of Ru(III)/Os(VIII)/Pd(II)/Pt(IV)-catalysedoxidation of Gly-Gly by CAT in NaOH medium were found tobe different and the catalytic efficiency of these platinum groupmetal ions increases in the order: Os(VIII)> Ru(III)> Pt(IV)> Pd(II).This trend may be attributed to different d-electronic configur-ation of the catalysts. Further, the rates of oxidation of Gly-Gly forall the catalysed reactions have been compared with uncatalysedreaction and found that the catalysed reactions are 7- to 24-foldfaster. Catalytic constants and activation parameters withreference to each catalyst have been computed. Based on theobserved experimental results, detailed mechanistic interpret-ation and the related kinetic modelling have been worked out foreach catalyst. It can be concluded that Ru(III)/Os(VIII)/Pd(II)/Pt(IV)


acts as efficient catalysts in the oxidation of Gly-Gly broughtabout by CAT in alkaline medium.

Acknowledgements

We gratefully acknowledge the UGC-DRS programme of ourDepartment for the encouragement and highly thankful toProfessor B. S. Sheshadri and Professor N. M. Nanje Gowda fortheir helpful discussions.

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